The invention relates to a dielectric barrier discharge lamp and a lighting system having such a lamp and an electric power supply unit.
The term xe2x80x9cdielectric barrier discharge lampxe2x80x9d in this case covers sources of electromagnetic radiation based on dielectrically impeded gas discharges. The spectrum of the radiation emitted by the gas discharge can in this case comprise both the visible region and the UV (ultraviolet)/VUV (vacuum ultraviolet) region and the IR (infrared) region. Furthermore, it is also possible to provide a fluorescent layer for converting invisible radiation into visible radiation (light).
The discharge vessel is usually filled with a rare gas, for example xenon, or a gas mixture. What are termed excimers are formed during the gas discharge, which is preferably operated by the use of a pulsed operating method described in U.S. Pat. No. 5,604,410. Excimers are excited molecules, for example Xe2*, which emit electromagnetic radiation upon return to the generally unbonded ground state. In the case of Xe2*, the maximum of the molecular band radiation is approximately 172 nm.
A dielectric barrier discharge lamp necessarily has at least one so-called dielectrically impeded electrode. A dielectrically impeded electrode is separated from the interior of the discharge vessel by the use of a dielectric barrier. By way of example, this dielectric barrier may be designed as a dielectric layer which covers the electrode, or formed by the discharge vessel of the lamp itself, specifically if the electrode is arranged on the outer wall of the discharge vessel.
Because of the dielectric barrier, the operation of such lamps requires a time-variable voltage between the electrodes, for example a sinusoidal AC voltage or pulsed voltage as disclosed in U.S. Pat. No. 5,604,410.
In the case of dielectric barrier discharge lamps, the first ignition or ignition after lengthy operating pauses is frequently difficult, in particular after lengthy storage of the lamps in the dark. As a rule, a substantially higher voltage is required than in normal operation. Moreover, upon first ignition a filamentary partial discharge frequently occurs which is undesired, since its useful radiation emission is inefficient by comparison with that of the discharge form disclosed in U.S. Pat. No. 5,604,410.
U.S. Pat. No. 6,097,155 has already disclosed a dielectric barrier discharge lamp having an elongated discharge vessel and having elongated dielectrically impeded electrodes arranged on the inside of the discharge vessel wall along the longitudinal axis.
A high-power radiator based on dielectrically impeded discharge is disclosed in U.S. Pat. No. 5,432,398 in the form of a coaxial double-tube arrangement. An outer electrode in the form of a wire mesh extends over the entire circumference of the outer quartz tube. A helical inner electrode is pushed into the inner quartz tube. The interior of the inner quartz tube is filled with a cooling liquid that has a high dielectric constant and, in addition to serving the purpose of cooling, also serves to couple the inner electrode to the inner quartz tube. A multiplicity of discharge channels form between the electrodes upon the application of an AC voltage in the space between the two tubes, the discharge space. For the purpose of improving the ignition behavior during the first ignition or after lengthy operational pauses, means are provided that force an initial ignition by means of local field distortion or field prominence at a point in the discharge space. The reliable ignition of the entire discharge volume is then forced by the UV radiation produced in this case and the charge carriers of this local discharge. The following are disclosed as suitable means for the field distortion: a dent in the inner or outer tube that reaches approximately up to half the gap width to the respective other tube; a sphere of dielectric material in the discharge space; a quartz droplet fused onto the inner surface of the outer tube or the outer surface of the inner tube.
It is the object of the present invention to provide a dielectrically impeded barrier discharge lamp which demonstrates improved ignition behavior.
This object is achieved by means of a dielectric barrier discharge lamp having an elongated discharge vessel defining a longitudinal axis, and having elongated dielectrically impeded electrodes arranged on the discharge vessel wall along this longitudinal axis, and having at least one electrically conductive means that extends with reference to the longitudinal axis only over a subregion of the discharge vessel wall and that is arranged on the discharge vessel wall to support the ignition of the dielectrically impeded discharge.
Also claimed is a lighting system having the above-named dielectric barrier discharge lamp and having a voltage source with two poles which can provide a pulsed-voltage sequence at these two poles, electrodes being connected to the two poles.
The dielectric barrier discharge lamp according to the invention has at least one electrically conductive means for supporting the ignition of the dielectrically impeded discharge that is arranged on the discharge vessel wall and extends with reference to the longitudinal axis only over a subregion of the discharge vessel wall.
It is assumed according to the present state of knowledgexe2x80x94without hereby intending to fix the theoretical interpretationxe2x80x94that this means permits an initial ignition between this means and at least one dielectrically impeded electrode more specifically at voltages that are already lower than without this means. This initial ignition then effects an ignition of the actual discharge between the dielectric electrodes. In addition, the means greatly reduces the probability of the undesired occurrence, mentioned at the beginning, of the filamentary partial discharge.
In a preferred embodiment, the dielectric barrier discharge lamp has inner electrodes, since this embodiment in accordance with U.S. Pat. No. 6,097,155 has proved to be particularly efficient. In this case, the dielectrically impeded electrodes are implemented by means of elongated electrodes that are arranged on the inside of the wall of the discharge vessel and are covered by a dielectric layer. The electrically conductive means is arranged on the outside of the wall of the discharge vessel.
This embodiment has the additional advantage that the means can be applied from the outside, that is to say after fabrication of the discharge vessel. Suitable in this case as electrically conductive means is, inter alia, a ring or part of a ring, in particular made from metal, which can also be mounted subsequently on the elongated discharge vessel, particularly in the form of a circular tube. Moreover, it is also possible to conceive further refinement of the means which fulfill the above-named purpose, for example a filament or spring tightly wound around the discharge vessel. Finally, a differently shaped planar refinement of the means is also possible in principle, for example a metal sheet of rectangular, round or oval shape, although further arrangements for fastening the means on the wall of the discharge vessel are to be taken in some circumstances. This can be avoided when the means is implemented by a corresponding conductive coating, for example a metal solder layer.
The width of the means along the longitudinal axis of the discharge vessel is typically between approximately 1 mm and a few 10 mm, particularly between 3 mm and 15 mm. It has proved that this is sufficient as a rule for reliable ignition, on the one hand, and that the light emitted by the lamp is still shaded to a relatively small extent, on the other hand. Moreover, the means is preferably arranged at one end of the discharge vessel. It has proved to be advantageous in this case when the means overlaps one end of the elongated electrodes. An overlap of a few mm, in particular approximately 1 mm, is already sufficient. However, the means can also overlap the elongated electrodes over its entire width.
In the case of very long lamps, it can possibly also be advantageous to provide two means, for example one at each end of the lamp, or else several means distributed along the lamp axis, in order to ensure rapid and uniform ignition of the entire lamp.
In a further preferred embodiment, the lamp has a base at at least one end, the means being integrated in the base.
Although the electrically conductive means can also be at a floating electric potential, it has proved to be favorable when the means is connected to ground potential, preferably to the plane potential of the voltage source supplying the lamp. The connection to plane potential has the advantage that defined voltage conditions are set up between the means and electrodes.
In order to make up a complete lighting system, the electrodes of the dielectric barrier discharge lamp according to the invention are connected to the associated poles of a voltage source. The means is connected to constant potential, with reference to the time-variable voltage at the poles of the voltage source. The voltage source is preferably designed in such a way that it can provide a pulsed-voltage sequence at its poles. Reference is made to U.S. Pat. No. 6,323,600 for further details on this. It is particularly preferred to design the voltage source in such a way that the voltage source can provide a symmetrical pulsed-voltage sequence with reference to its plane potential, the means being connected to the plane potential. The use of a symmetrical voltage has the advantage here, inter alia, that no undesired capacitive currents flow via the means to the ground line.